DE 35 44 786 A1 describes an arrangement for determining the concentration of gases by exploiting the dispersion of the velocity of sound, in which the phases of the two different measurement frequencies are directly compared with each other in the phase detector and the phase difference is used as a measure of the gas concentration.
US 2011/0314897 A1 describes a sensor chip for gas, which comprises cells for transmitting and receiving ultrasound and is designed to cover a broad range of frequencies and to measure a concentration of at least one gas component based on at least two responses within the range.
DE 29 45 172 A1 describes a method for determining the concentration of a gas, in particular the carbon dioxide content, in a gas mixture, wherein the sharply modified sound velocity of the gas in a dispersion area is used for determining the gas content by means of dispersion measurements on the gas mixture.
U.S. Pat. No. 6,487,916 B1 describes a method for determining a flow velocity of a fluid. A fluid is provided, which flows in a given direction. A first pulse is sent perpendicular to the given direction over a first known distance within the fluid and received. A second pulse is sent at an oblique angle to the given direction over a second known distance within the fluid and received. The respective speeds of the first and second pulses and their difference are determined. The difference is used to determine the flow velocity. The flow velocity is compared with a target flow speed.